A battery housing an electrolyte in an enclosed battery case is typically provided with a safety valve that ruptures when the internal pressure of the battery case exceeds a predetermined value so that the internal pressure of the battery case is released to the outside in order to prevent the battery from exploding due to an increase in internal pressure of the battery case. A method disclosed in Japanese Patent Application Publication No. 2007-141518 A and so on, for example, may be cited as a conventionally employed method of manufacturing battery safety valve of this type.
In this conventional method, a thin annular portion constituting an edge portion of a safety valve is formed on a metal sheet serving as a lid of a battery case by disposing the metal sheet between an engraving punch having an annular projecting portion and a planar die, and pressing the annular projecting portion of the engraving punch against the metal sheet while supporting the metal sheet using the planar die. When the internal pressure of the battery case exceeds a predetermined value, the thin annular portion, which is formed to be thinner than other parts, splits such that the entire safety valve ruptures.
As described above, the thin annular portion is the part that is intended to split first when the internal pressure of the battery case exceeds a predetermined value, and therefore the thickness thereof is reduced to approximately 0.010 mm, for example. In other words, the punch and the die used to form the thin annular portion are processed extremely finely.
In the conventional method for manufacturing a battery safety valve described above, the thin annular portion is formed on the metal sheet by pressing the annular projecting portion of the engraving punch against the metal sheet while supporting the metal sheet using the planar die, and therefore the thin annular portion is essentially formed by deforming the metal sheet using only the annular projecting portion of the engraving punch.
A soft metal such as aluminum is conventionally used for the metal sheet on which the safety valve is molded. Recently, however, the use of stainless steel has been proposed with a view to improving the corrosion resistance and strength of the battery case.
When the metal sheet is made of aluminum, the thin annular portion can be processed precisely due to the softness of the metal. Moreover, since the load exerted on the die assembly is small, processing can be performed without problems. Stainless steel, on the other hand, is a hard material having great strength, and therefore, when an attempt is made to process the thin annular portion of the safety valve using the conventional method, cracks are more likely to appear during the processing. Further, the thin portion cannot be processed precisely due to work hardening.
Furthermore, with the conventional method, the load generated during formation of the thin annular portion is concentrated on the annular projecting portion of the engraving punch, leading to a reduction in the lifespan of the die assembly.